1. Field of the Invention
The present invention relates to an optical memory element for recording, reproducing, erasing information using a laser beam and its manufacturing method, and more particularly, to an optical disc having favorable antistatic properties or resistance to marring and its manufacturing method.
2. Description of the Related Art
Recently, research and development is briskly underway for an optical memory element with a high recording density and a large capacity. A glass and transparent resins are suitable for substrates of the optical memory element. In particular, polycarbonate is preferably suitable among the resins, because it has a good productivity and moisture resistance.
However, dust easily sticks to polycarbonate substrates because they are easily electrically charged owing to their high surface resistivity of 10.sup.16 .OMEGA.. In an optical disc memory, a beam is focused on a recording film in the order of .mu. m through the transparent substrates of approximately 1.2 mm thickness made of glass or transparent resins. Therefore, generally the focused beam may not be possibly affected by the dust on the transparent substrate. But in actual case, the dust causes a problem on recording and erasing of a magneto-optical disc, though it does a little on playback of a Compact Disc-as mentioned above. It is because recording and erasing consumes a lot of energy as compared with playing back, and the dust scatters the beam on the substrate surface, which causes a recording or/and erasing error as a result of energy power shortage.
In order to prevent the beam from scattering as shown FIG. 6, a proposal is made to cover an optical disc with an antistatic layer 42 where a metal oxide is dispersed as electro-conductive filler in acrylic resin layer (Japanese Unexamined Patent Application No. 158643/1989). In addition, a recording layer 44 is formed on a back surface of the polycarbonate substrate 41.
But FIG. 6 is a schematic view showing the optical disc. In actual case, both surfaces of the polycarbonate substrate 41 are not even as shown in FIG. 7. More specifically, an annular ridge 48 and a groove 46 are formed at an inner circumference on the front and back surfaces of the polycarbonate substrate 41, respectively corresponding to a gap or a swelling in a joint between parts of a mold for injection machine. In addition, a height of the ridge 48 and a depth of the groove 46 are not uniform in the circumferential direction. A centering part 45 called a center hub is bonded on an inner hole 43 of the optical disc. The ridge 48 and the groove 46 are formed in the vicinity of outer circumference of the centering part 45.
An antistatic layer is spin-coated on the surface of the polycarbonate substrate 41. A density of the antistatic layer changes in the circumferential direction corresponding to a distribution of ridge height 48 or the groove depth 46. As a result, the antistatic layer changes in thickness in the circumferential direction, resulting in a shift in the focused optical beam offset, disturbance of a servo signal for recording, reproducing and erasing, and decrease in the power of the optical beam.
When an antistatic layer 49 is formed outside the ridge 48 as shown in FIG. 7 in order to prevent wide distribution of the film thickness of the antistatic layer, dust is gathered at an exposed portion 47 free of the antistatic layer 49. Although the exposed portion 47 serves as a reference surface when the optical disc is mounted on a drive apparatus, the reference surface cannot be provided or the polycarbonate substrate 41 is damaged if the dust exists on the exposed portion 47.
In addition, the centering part 45 and the polycarbonate substrate 41 are formed of the same material so that their adhesive state will not suffer from a different expansion nor shrinkage by heat. More specifically, the centering part 45 is also formed of polycarbonate. Since an antistatic means is not taken in the prior art, the dust also sticks to the centering part 45. As a result, the dust is piled together on the surface of the optical disc owing to peeling from centering part 45 by a vibration of the disc or the like and tends to disturb the servo signal and decrease the power of the optical beam.